1. Field of the Invention
This invention relates to gas turbine engines and more particularly to apparatus forming a portion of a flow path wall for the working medium gases in the turbine section of the engine.
2. Description of the Prior Art
In a gas turbine engine of the type referred to above, pressurized air and fuel are burned in a combustion chamber to add thermal energy to the gases flowing therethrough. The effluent from the chamber comprises the working medium gases and is flowed axially downstream in an annular flow path through the turbine section of the engine. A first row of nozzle guide vanes at the inlet to the turbine direct the medium gases onto a multiplicity of blades which extend radially outward from the engine rotor. An annular shroud which is supported by the turbine case surrounds the blade tips to confine the medium gases to the flow path. A second row of nozzle guide vanes is positioned axially downstream of the blades and the shroud to redirect the medium gases to a preferred course. During operation of the engine, the shroud adjusts downstream into abutting relationship with the vanes to prevent the leakage of air therebetween.
The blade tip shrouds are commonly segmented where large variations in thermal expansion between the shroud and the supporting case are expected. A circumferential gap between adjacent segments is provided to allow independent expansion of the case and the shroud segments without inducing local stresses. The number of segments comprising an individual shroud is set with regard to the expected thermal gradient in the radial direction across the segment. The temperature of the shroud during operation of the engine is normally highest adjacent the medium flow path and causes the segments to flatten from the initial arcuate geometry. Severe flattening in many constructions causes the segments to bind in their respective supporting tracks and prevents their rearward adjustment into abutting relationship with the platforms of the downstream vanes.
The vanes which are located immediately downstream of the shroud are also grouped into segments hereinafter called vane clusters. Each vane cluster comprises a plurality of vanes which are generally welded together at their platforms. The joining of two or more adjacent vanes reduces the amount of vane twisting, within the clearance required for assembly, which normally occurs as a result of thermal gradients across each vane. The number of vanes which can be joined in each cluster is limited by the stiffness of the resulting assembly which, when overly stiff, induces high thermal stresses in the vane material during operation of the engine. No correlation exists in the prior art engines between the number of shroud segments and the number of vane clusters immediately downstream thereof.
Cooling air is commonly flowable through conduits in the turbine section to prevent destructive overheating of the various turbine components and to control the diametral growth of the turbine case. One such conduit is commonly formed between the turbine case and the abutting shroud and vane platforms which are spaced radially inward therefrom. Leakage from the conduit is minimized in constructions, such as that shown in U.S. Pat. No. 3,860,358 to Cavicchi et al. entitled "Turbine Blade Tip Seal", which discourage binding of the shroud segments in their supporting tracks and permit the rearward adjustment of the individual segments into abutting relationship with the platforms of the downstream vanes.
Notwithstanding the rearward adjustment of the shroud segments substantial leakage continues to occur between the shrouds and vane platforms. Present efforts are directed toward developing improved apparatus which will further inhibit the flow of cooling air between the shroud segments and the downstream vanes.